Cyclic siloxanes are often used as starting monomers to form polysiloxanes. Generally polysiloxanes can be formed by the ring opening polymerization of cyclic siloxanes and a suitable end-capping unit. The reaction is carried out in the presence of a catalyst.
Various catalysts are known for the polymerization of cyclosiloxanes. Examples are alkali metal hydroxides, alkali metal alkoxides or complexes of alkali metal hydroxides and an alcohol, alkali metal silanolates, and phosphonitrile halides (sometimes referred to as acidic phosphazenes). Such polymerizations can be carried out in bulk, in solvents (such as non-polar or polar organic solvents) or in emulsion. Phosphazene bases and carbenes have been described as suitable catalyst for the ring opening polymerization of siloxanes. An endblocking agent may be used to regulate the molecular weight of the polymer and/or to add functionality. Polymerisation may be terminated by using a neutralizing agent which reacts with the catalyst to render it non-active. In most cases catalyst residues remain in the polymer product and are desirably removed, such as by filtration.
Solid type catalysts can be used as a catalyst to synthesize polydimethylsiloxane (PDMS) fluids and PDMS functional fluids by ring opening polymerization. Solid type catalysts exhibit high catalytic activity, but generate solid waste. The solid waste is typically incinerated. Additionally, the solid waste contains a significant fraction of the product, which decreases the overall yield and can increase production costs.
Curable compositions such as moisture curable compositions also employ catalysts to promote curing and formation of the polymer network. Metal catalysts are typically used as condensation cure catalysts to accelerate the moisture-assisted curing of polyorganosiloxanes and non-silicone polymers having reactive terminal silyl groups in room temperature vulcanizing compositions. Organotin, such as dibutyltindilaurate (DBTDL), is commonly used as a catalyst in such compositions. Environmental regulatory agencies and directives, however, have increased or are expected to increase restrictions on the use of organotin compounds in formulated products. For example, while formulations with greater than 0.5 wt. % dibutyltin presently require labeling as toxic with reproductive 1B classification, dibutyltin-containing formulations are proposed to be completely phased out in consumer applications during the next four to six years.
The use of alternative organotin compounds such as dioctyltin compounds and dimethyltin compounds can only be considered as a short-term remedial plan, as these organotin compounds may also be regulated in the future. Desirably, substitutes for organotin catalysts should exhibit properties similar to organotin compounds in terms of curing, storage, and appearance. Non-tin catalysts would also desirably initiate the condensation reaction of the selected polymers and complete this reaction upon the surface and may be in the bulk in a desired time schedule. There are therefore many proposals for the replacement of organometallic tin compounds with other metal-based compounds. These compounds comprise metals such as Ca, Ce, Bi, Fe, Mo, Mn, Pb, Ti, V, Zn, and Y. These other metals have specific advantages and disadvantages in view of replacing tin compounds perfectly. Non-metal catalysts are also of interest. Therefore, there is still a need to address the limitations of possible metal compounds as suitable catalysts for condensation cure reactions. The physical properties of uncured and cured compositions also warrant examination, in particular to maintain the ability to adhere onto the surface of several substrates.